Regenerative filter



Patented Sept. 29, 1936 I REGENERATIVE FILTER Ludwig W. Blau and James S. Watt, Houston, Tex, assignors to Standard Oil Development Company, a corporation of Delaware Application June 22,1934, Serial .No. 731,946

I 10 Claims. (Cl. 178-44) This invention relates to an improvement in may be resistors, inductance coils, condensers, or filters. combinations in series or in parallel of resistors, It has been known how to combine electric netand/ or inductance coils and/or condensers. The works so as to render them frequency selective. particular impedance used depends upon the kind Thus it is possible to make low pass filters which of filter it is desired to build. 5 passlow frequencies and discriminate against An amplifier is operatively connected across high frequencies. a High pass filters, on the other the input and output impedances I and 2. The hand, pass or high frequencies and discriminate amplifier comprises a vacuum tube I6. The vacagainst the low frequencies. Such a filter is the uum tube includes a grid I1 and filament I8 which 10 subject of the copending applicatio of Ludwig are connected across the output impedance 2. A 10 W. Blau, U. S. Serial No. 659,822, entitled Methp ate 0 of the vacuum tube is with the filament d d apparatus for eliminatingjow frequenl8 connected across the input impedance I of the cies, filed May 6, 1933. filter. A, B, C and other battery connections When filters are to be included in electric l are not shown because their insertion into a cirrecording systems where high accuracy is cuit such as described here is familiar to those 15 quired, it becomes necessaryto damp out all oscil- Skilled in the The c um tube I6 while lations originating in the filter itself. This is illustrated as a ee e ectrode tube may be any done successfully in the filter which is the subject foul electrode p e ctrode tube. of theabove mentioned patent application. How- Circuit p s as o ows: The oscillator ever, the introduction of damping renders the 3 lmpresses a voltage across the input impedance 20 cut-off of t filter 1 Sharp 50 t more tages I which is transmitted through the filter to the u t be dd d t k up for t i means output impedance 2. The frequency content is that the filter becomes larger and more expensive Changed by the filter, Some Of e eq encies to construct. The invention which is the subject being p d While others are discriminated of this application permits of building a damped against. Thus if it is assumed for a moment 25 filter of a few stages with a sharp cut-offheretothat a g Pass fi e s being sed with a cut fore obtained only by using many more stages. off at 40 cycles. t frequencies higher than 0 The invention will be fully understood from the will pass t o h the filter with little or no atfollowing description taken in connection with tenuation, while the lower frequencies are atthe accompanying drawing in which 1atter tenuated. Thus the filter may pass all 50 cycle 36 Fig. 1 is a generalized schematic illustrating energy, 95% of the 40 cycle, 75% of the 30 cycle, one form of the invention; of 20 cycle and of 10 cy n r y- The Fig. 2 is a generalized schematic illustrating a addition of more stages would make the out 01f modified form of the invention; sharper; thus by using twice as many stages one 5 Fig. 3 is a diagrammatic representation of a would obtain the following P a es at the 35 specific form of circuit embodying the invention, difierent frequencies:

and

Fig. 4 is a diagrammatic representation of an gig; g zg by arrangement of apparatus for reflection shoot- 40 ing. 40

Referring particularly to Fig. l of the drawing Table I 50 reference numeral I designates an input imped- 40 9o ance and numeral 2 designates an output imped- 2?, i; ance. An electric circuit connects the imped- 10 0.25

ances. The input impedance I is connected to 45 an oscillator 3 or other source of alternating cur- In the last mentioned filter the cut off would rent. A plurality of parallel branches'5, 6 and be much sharper, but the addition of the stages I are provided in the electric circuit. Each would be costly and the filter would be larger. branch'includes an impedance Ill. An imped- According to the invention, the voltage across ance I2 is provided in the circuit between the i pe a e 2 is impressed upon the grid of the 50 impedance I and the branch 5. An impedance vacuumtube l6 where it is amplified and is in I3 is provided in the electric circuit between the turn impressed upon the input impedance I,

branch I and the impedance 2. Impedances I4 From the input impedance I the amplified voltare -provided in the circuit between adjoining age is transmitted again to the impedance 2. branches. The impedances I, 2, In, I2, I3 and I4 It is then reimpressed upon the grid of the vac- 55 each round trip the filter discriminates against all frequencies in the attenuation band so that the energy content of these frequencies is reduced each time. g 7

Thus .if we assumethat say a fraction 10 of a certain frequency impressed upon the input impedancel is transmitted to the output impedance 2 and'reimpressed upon. the input, then on the first trip the volta e of this frequency'impressed' on impedance lis 1; on the second trip 10, on

the third trip 11 on the fourth trip p and on the n th in Then after n+1 round trips-we have 'for the sum of all these fractions .Now since 11 is less than unity, p approaches zero as limit and for a large number of round trips wecan write 7 T. R'. Thesum S gives the amplification, due to regeneration, 'of the frequency of which the filter passes a fraction p of vthe input voltage. But, in

theexample assumed above, we had the following values of p. for the different frequencies: 1

Freq. 1)

. a V 50 '1 I a r r 40 .95

Substituting these values of p' in the equation for S.- above, we have the table for the'amplificareducedit to 04%. The cut off has been made" tion .of' the different frequencies:

- :Since an infinite amplification is impossible one would set'the regeneration possible at 50 cycles to a slightly smaller value than unity; assuming .999 and higher'valu'es can be used in practice, the amplification at 50 cycles would be 1000times. Becalling now the Values given in Table II which' give the characteristics of the filter without regeneration, We find that whereas without regeneration a frequency of 30 cycles was as strong as 50 cycles, thelregenerative filter has as sharp as would be obtained with'a filter with more than 10 times. as many sections. It is not proposed v.to limit the invention to a three section' filter; such ashas been shown in Fig. l.

I v If the filterused is ofth'e high pass type with a cut off frequency at say cycles, then the -volt-, age resulting across the. output impedance 2 due totheimpression of a frequency of 5 cyclesacross the input impedance I will be Very small or even negligible on account of the filter action of the filter. This low voltage across the output impedance 2 is, however, impressed across the grid and filament of the vacuum tube It and for this frequency of five cycles, or for any other frequency below the cut off frequency of the filter, the grid and filament of the vacuum tube will therefore be practically shorted so that there will be no amplification in the plate current due to this low frequency. The voltage resulting across the output impedance 2 due to a high frequency impressed across the input impedance I may be very large. Any such voltage impressed upon the grid and filament of the vacuum tube IE will result in'an amplification by the vacuum tube [6 of the voltage across the output impedance 2' by. a .factor whichdepends upon the construction of, and the particular voltages impressed upon the vacuum tube Hi. It is evident therefore that all frequencies higher than the cut off frequency will have their amplitudes magnified by a predetermined factor depending upon the type of vacuum tube selected. In the case of a low pass filter the reverse would be true, the low frequencies would be amplified while the high frequencies which thefilter attenuates suffer deleterious discriminations. In the case of band pass filters, thegfrequencies lying in the pass bandwould be amplified while those lying in the attenuation band would be reduced in amplitude. While the source of alternating current has been described as an oscillator 3, it will be understood that any other electrical input such as abroadcasting microphone, aphonograph pick-up or anyother electrical input canv be substituted for the oscillator I .and someof the advantages of the invention; will be retained.

Referring particularly to Fig. 2, a modified form of the inventioniisshown inwhich the filter is identical with the filter illustrated in Fig. 1 and like. reference numerals are used to designate like parts. In this figure, however, the grid I1 and filament H! of .the vacuum tube [6 are connected across the impedance ill of the shunt arm 7. The plates 20 and filament [8 are connected across the. impedance IO'of the shunt arm 5. Alternatively the grid and filament of the vacuum tube can be connected across either the output impedance or any one ofthe later shunt arm impedances, and the plate 20 can be connected across the input impedance or any one of the earlier impedances and some of the advantages of the invention will be retained.

Referring to Fig. 3,'the invention is illustrated as applied to. one of the filters which constitutes the subject matter of the copending application above referred to. Infthis figure reference numeralIE-designatesan input resistance, such as the resistance of an electrical seismograph, and numeral .zlirepresents an output resistance. An electric circuit connectsthe resistances. A plurality of parallel branches 21,28, 29, 30 and 3| are providedqin the circuit. Each branch includes a condenser 33 together. with an induce tance and resistance 35 of a choke coil. A condenser 31 is provided in the circuit between the resistances 25-and the branch 21; and a condenser 38 is provided in the circuit between the branch :3 l and the output resistance 26. Ardamping resistance-40 ,and'acondenser 4| are connected in seriesin the circuit between each of the adjoining brancheszl, 28, 29,30 and 3|. .In the embodiment illustrated; the vacuum tube l6,.identical, in "all, respects, with the 'yacuum tubes previously known filters having the same number of I viously described, is connected through its grid and filament I8 across the output resistance 26. The plate [8 of the vacuum tube is connected with the filament across the branch 21.

Referring to the embodiment illustrated in Fig. 4, a preferred arrangement of apparatus for reflection shooting is shown in which reference numeral 43 designates the coil of a magneto-electrical seismograph such as is described in the copending application Serial No. 626,132 of Ludwig W. Blau et al., entitled seismograph, filed July 29, 1932. The coil 43 is connected across the grid and filament of a vacuum tube 44 which may be a three-electrode or screen grid tube. The

vacuum tube 44 is connected to the input resistance 25 of the circuit previouslydescribed in connection with Fig. 3. The output resistance 26 of the circuit is'connected to a vacuum tube amplifier 45 which in turn is connected to an oscillograph 46 which records the seismogram. The vacuum tube I6 is connected through its grid and filament across the output resistance 26. The plate of the vacuum tube is connected with the filament across the input resistance 25. The vacuum tube 44 if used is either of the same type as the vacuum tube H3 or if dilferent tubes are used, the two types must require the same B voltages because of the fact that both tubes are supplied from the same source of B voltage. The vacuum tube 44 is used where the input currents resulting from the inductance coil, or any other electrical input which may be inserted in its place, are very feeble and it is deemed desirable to amplify these feeble oscillations prior to their impression upon the filter.

By the construction described, undesired frequencies are not amplified while desired frequencies lying in the pass band of the filter are amplified. A relatively sharp cut-off is obtained as compared with that obtained with the presections. Alternatively, the same cut-off can be obtained as formerly by using a smaller number of sections.

Various changes may be made within the scope of the appended claims in which it is desired to claim all novelty inherent in the invention as broadly as the prior art permits.

We claim:

1. A selective amplifier, comprising an electric circuit constituting a filter with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the circuit, and amplifying means one end of which is connected across an impedance near the output end of the circuit and the other end of which is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy from adjacent the output end to adjacent the input end for further filtering.

2. A selective amplifier, comprising an electric circuit constituting a filter with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the. circuit, and a vacuum tube including a grid and filament connected across an impedance near the output end of the circuit and including a plate which with the filament is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy from adjacent the output end to adjacent the input end for further filtering.

3. A selective amplifier, comprising an electric circuit constituting a filter with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the circuit, and amplifying means one end of which is connectedacross the impedance at the output end of the circuit and the other end of which is connected across the impedance at the input end of the circuit so as to permit the passage and amplification of energy from the output end to the input end for further filtering.

4. A selective amplifier, comprising an electric circuit constituting a filter with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the circuit, and a vacuum tube including a grid and filament connected across the impedance at the output end of the circuit and including a plate which with the filament is connected across the impedance at the input end of the circuit so as to permit the passage and amplification of energy from the output end to the input end for further filtering.

5. A selective amplifier, comprising an input impedance, an output impedance, an electric circuit constituting a filter connecting the impedances with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the circuit, and amplifying means one end of which is connected across an impedance near the output end of the circuit and the other end of which is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy from adjacent the output end to adjacent the input end for further filtering.

6. A selective amplifier, comprising an input impedance, an output impedance, an electric circuit constituting a filter connecting the impedances with parts of the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across the cir-, cuit, and a vacuum tube including a grid and filament connected across an impedance near the output end of the circuit and including a plate which with the filament is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy from adjacent the output end to adjacent the input end for further filtering.

'7. A selective amplifier, comprising the inductance coil of a seismograph connected across the grid and filament of a vacuum tube, an input impedance connected to the plate resistor of the vacuum tube, an output impedance, a filter connecting the impedances with parts of the filter aperiodically damped for the natural frequencies of the filter, impedances shunted across the circuit of the filter, amplifying means one end of which is connected across an impedance near the output end of the circuit and the other end of which is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy from the output end to the input end for further filtering, an amplifier connected across the output impedance, and an oscillograph connected to the .amplifier.

8. A selective amplifier, comprising the inductance coil of a seismograph connected across the grid and filament of a vacuum tube, an input impedance connected to the plate resistor of the vacuum tube, an output impedance, a filter connecting the impedances with parts of the filter aperiodically damped for the natural frequenthe circuit; of the filter, a vacuumtube including a. grid and. filament-connectedacross an 'impedance near the output end of the circuit and including a'plate which with the filament is connected across an impedance'vrelatively near the input end of thecir'cuitso as to permit. the pas sage and amplificationof energy from theoutput end to the input endfor further filtering, an amplifier'connec'ted across the output impedance, and an oscillograph connected to the'amplifier.

9. A selectiveamplifier, comprising an electric circuit, constituting a'filter with parts of :the filter aperiodically damped for the natural frequencies of the filter and including impedances shunted across-the vcircuit, and amplifying means one end of which is connected across one of the impedances and the other end of which is connected across an impedancerelatively near the 2,055,619 cies of thefilter, theimpedances shunted across inputend of the circuit so as to permit the passage and amplification of energy in the reverse direction to its passage through the filter for further filtering.

a 10. A selective amplifier, comprising an electric circuit constituting a filter with parts of the filter aperiodically damped for the natural frequenciesof the filter and including impedances shunted'ac'ross'the circuit, and a vacuum tube including a grid and filament connected, across an impedance and including a plate which with the filament is connected across an impedance relatively near the input end of the circuit so as to permit the passage and amplification of energy in the reverse direction to its passage through the filter for further filtering.

LUDWIG W. BLAU. JAMES S. WATT. 

